Field of the Disclosure
The disclosure generally relates to trapezoidal thread forms and more specifically to trapezoidal thread forms having a wear indicator, and pressure connection fittings comprising such thread forms.
Related Technology
Threaded connections are often used to join two members to one another. For example, a nut and bolt may be joined by a threaded connection. Likewise, two pipe sections may be joined by a threaded connection. Threaded connections generally include a male part and a female part. The male part includes threads formed on an outer surface and the female part includes threads formed on an inner surface. The threads include an elevated portion or ridge surrounded on either side by recessed portions or valleys. The elevated portions and recessed portions are generally helical in shape. When connected, the elevated portions of the male threads generally seat within the recessed portions of the female threads. By twisting or turning the male and female parts relative to one another, the elevated portions may be advanced through the recessed portions as the male and female parts advance longitudinally with respect to one another.
The threads themselves may take on many different cross-sectional shapes. For example, common fasteners, such as nuts and bolts, may have a V-shaped thread where the recessed portions have a cross sectional shape that looks like a “V” and the elevated portions have a cross sectional shape that looks like an inverted “V.” While V-shaped threads may reliably secure a male part to a female part, the V-shape focuses material stresses at the sharp edges, which can lead to failure. Furthermore, the V-shape means that the part of the elevated portion that is seated most deeply within the recessed portion is also the thinnest or weakest part of the thread, which can also lead to failure. Additionally, the sharp peak of the elevated portion is more susceptible to being damaged by impacts, which can deform the top of the elevated portion significantly enough that it cannot seat in the recessed portion.
Another type of thread is the square thread. These threads have a square-shaped cross section. Square threads are generally stronger than V-shaped threads as the elevated portions have a thicker cross section near the ends. While square threads are stronger than V-shaped threads, square threads are more difficult to manufacture and have a relatively short lifespan.
A trapezoidal thread is an improvement over square threads. Trapezoidal threads have a trapezoid-shaped cross section. Trapezoidal threads have high strength, are generally easy to manufacture, and can compensate for wear, which leads to longer life spans. Trapezoidal threads are often used on power screws, or other applications that require strong and accurate connections.
One type of trapezoidal thread is known as an Acme thread. The Acme thread includes a 29° thread angle and a thread height that is half of the pitch. The ridge and valley of the Acme thread are relatively flat, which makes machining the threads easier. The thread also has a wider base, which increases strength of the connection. The Acme thread also allows use of a spilt nut, which can compensate for some thread wear. Due to the thicker bases and wider channels, Acme threads also work well in environments having contaminants, such as dirt, that can get caught between the threads. Acme threads are commonly found on hose end-connection, filling valves, valve stems, and other common locations where quick connection and disconnection is required and/or where large forces may be encountered. These threads are relatively durable, but like any other type of threaded engagement are susceptible to wear.
Any type of thread is susceptible to wear during use. Worn threads can decrease the strength of a threaded connection significantly. Due to the nature of threaded connections, wear is often difficult, or impossible, to detect. As a result, excessively worn threads may go undetected until failure.